Laser welding system and method using cooling mask to control the width of the weld

ABSTRACT

A laser welding method and system for joining portions of first and second workpieces of thermoplastic material that is partially permeable to a laser beam but absorbs radiation from the laser beam. The first and second workpieces, which are made of material that absorbs radiation from a laser beam, are clamped together. A mask is placed on a first surface of the first workpiece, the first surface being opposite the surface engaging the second workpiece. The mask is impermeable to a laser beam and forms a slot for passing a laser beam to the portion of the first surface of the upper workpiece exposed by the slot, so that heating and melting of the material of the workpieces is limited to the width of the slot. A laser beam is directed onto the slot and moved in a manner to illuminate the slot to melt and join the workpieces.

FIELD OF THE INVENTION

This invention relates generally to laser welding of thermoplasticmaterials.

BACKGROUND OF THE INVENTION

Various ways to weld clear thermoplastic sheets into packages are known.The packages are typically made of clear plastic sheets (e.g., blisterpackages) by using RF welding methods, which are limited to a smallnumber of thermoplastic materials (in practice, mostly PVC-based), whichhave the requisite electrical properties, such as a relatively highdielectric loss that provides the ability to convert alternatingelectric fields into heat. The process is not applicable or have verylimited applicability to the most common plastics, from polyolefines toPC to Polystyrene to Polycarbonate, which in turn severely limits thematerial selection available for blister packaging. With a currentcampaign to limit the use of PVC because of its environmentalhazard—recycling process for PVC is hazardous to various degrees due tothe material's inherent chlorine content, as well as the range ofchemical additives used for increasing material stability and variety inusage properties, the ability to utilize a wide range of thermoplasticsfor manufacturing of blister packages offers significant environmentalbenefits.

Another problem of RF welding is inflexibility of the process, as theseam is limited by the shape and size of the electrodes—each package'sshape would require the electrodes of a specific shape and size. Also,in practice the size of the package is limited by the required powercapacity, as the RF is known by very high power requirements to achievematerial melting. For example, for a perimeter seal of a 10 cm×15 cmmedical bag, the power requirements range from 1.7 to 2.9 KW (Plasticsand Composites Welding Handbook, Hanser Gardner Publications, Inc.,2003. p. 262).

Laser welding of plastics has established itself as a robust, flexibleand precise welding process, which is increasingly used to join plasticparts. It enables highly efficient and flexible assembly processes fromsmall-scale production of parts with complex and varying geometries to ahigh volume industrial manufacturing where it can be easily integratedinto automation lines.

Laser welding uses a laser beam to melt the plastic in the joint area bydelivering controlled amount of energy to a precise location. This isbased on the ease of controlling the beam size and the range of methodsavailable for precise positioning and moving the beam.

The process is based on the same basic requirements of materialcompatibility as other welding techniques, but is often found to be moreforgiving of resin chemistry or melt temperature differences than mostother plastic welding processes. Nearly all thermoplastics can be weldedusing a proper laser source and appropriate joint design.

Other objects and advantages of the invention will be apparent from thefollowing detailed description of illustrative embodiments, taken inconjunction with the accompanying drawings.

SUMMARY OF THE INVENTION

In accordance with one embodiment, a laser welding method is providedfor joining portions of first and second workpieces of thermoplasticmaterial that is partially permeable to a laser beam but absorbsradiation from the laser beam. The method comprises clamping togetherthe portions of said first and second workpieces to be joined, the firstand second workpieces being made of material that absorbs radiation froma laser beam; placing a mask on a first surface of the first workpiece,the first surface being opposite the surface engaging the secondworkpiece, the mask being impermeable to a laser beam and forming a slotfor passing a laser beam to the portion of the first surface of theupper workpiece exposed by the slot, so that heating and melting of thematerial of the workpieces is limited to the width of the slot;directing a laser beam onto the slot and moving the laser beam in amanner to illuminate the slot and to melt and join the first and secondworkpieces along the slot while the workpieces remain clamped together;and cooling the molten portions of the first and second workpieces tosolidify the joined portions of the workpieces and form a weld seamwhile the workpieces remain clamped together. The laser beam ispreferably an optical fiber laser beam having a wavelength of about 2microns.

The invention also contemplates a laser welding system for joiningportions of first and second workpieces made of thermoplastic materialthat is partially permeable to a laser beam but absorbs radiation fromthe laser beam. The system includes a pair of clamping plates positionedto engage opposite sides of the first and second workpieces when theworkpieces are adjoining each other with contacting surfaces; anactuator urging at least one of the clamping plates toward the otherclamping plate to press the workpieces together; a mask on the oppositeside of the first workpiece from the second workpiece, the mask beingimpermeable to the laser beam and forming a slot for passing the laserbeam to the portion of the first workpiece exposed by the slot, so thatheating and melting of the material of the workpieces is limited to thewidth of the slot; a laser beam source directing a laser beam into theslot; and a drive unit moving the laser beam to melt and join the firstand second workpieces along the slot while the workpieces remain clampedtogether. In one embodiment, the mask absorbs heat from the firstworkpiece to cool the portions of the first workpiece not exposed to thelaser beam. The laser beam is preferably an optical fiber laser beamhaving a wavelength of about 2 microns.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a perspective view of a laser welding arrangement for weldingtwo thermoplastic workpieces.

FIG. 2 is an enlarged sectional view taken along line 2-2 in FIG. 1.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

Although the invention will be described in connection with certainpreferred embodiments, it will be understood that the invention is notlimited to those particular embodiments. On the contrary, the inventionis intended to cover all alternatives, modifications and equivalentarrangements as may be included within the spirit and scope of inventionas defined by the appended claims.

Turning now to the drawings, a laser source 10 generates a laser that istransmitted through a fiber cable 11 to a scan head 12 attached to amount 13. The mount 13 is coupled to orthogonal gantries 14 and 15. Oneor more scanner mirrors within the scan head 12 are controlled by aprocessor-controlled drive unit 16 to direct a laser beam 17 downwardlyonto a stack 18 that includes two thermoplastic workpieces 21 and 22 tobe joined by welding. The drive unit 16 is controlled to adjust thepositions of the scanner mirrors to move the laser beam 17 in a mannerrequired to illuminate a prescribed weld zone on the top surface of thestack 18.

The top layer 19 in the stack 18 is a transparent glass plate 19 thathas a reflective-coated photo mask 20 on the bottom surface of the glassplate 19. The mask 20 forms a slot 20 a that permits the laser beam 17to reach the upper workpiece 21, which at least partially absorbs thelaser radiation. The mask 20 limits the exposed area of the workpieces21 and 22 to the desired weld zone defined by the slot 20 a, and thuslimits the melting of the upper workpiece 21 to the desired weld zone.In addition, the mask 20 functions as a heat sink and cools the surfaceof the workpiece 21 in the areas outside the weld zone. In analternative embodiment, the mask 20 is on the top surface rather thanthe bottom surface of the glass plate 19. One suitable material for themask is chrome plated on the surface of the glass plate 19.

The movement of the scanner mirrors is controlled by the processor thatcontrols the drive unit 16. When welding large parts, the movement ofthe laser beam 17 is controlled by driving the mount 13 along thegantries 14 and 15.

The basic welding technique used in the illustrated system providessignificant advantages over through transmission laser welding (TTLW),in which the workpieces are pre-assembled and clamped together toprovide an intimate contact between their joining surfaces. The laserbeam is then delivered to the interface of the workpieces' interfacethrough the upper transparent workpiece and is absorbed by the lowerabsorbing workpiece, which converts infrared energy into heat. Carbonblack and specially designed absorbers are blended into the resin of thelower workpiece, or applied to the surface, to enable infrared radiationto be absorbed in the lower workpiece. The heat is conducted from thelower absorbing workpiece to the upper workpiece to melt the upperworkpiece at the interface and form a bond. Precise positioning andclamping of the assembly ensures the intimate contact required for heattransfer between the parts.

Because the TTLW welding technique is dependent on the presence of anabsorbing agent in the lower workpiece, it limits the applicability ofthis assembly process for manufacturing of medical devices, electronics,some consumer goods and packaging applications where a “clear-to-clear”or a “clear-to-colored” assembly is required. However, a laser having awavelength of about 2 microns, commonly referred to as a “2 micronlaser,” is characterized by a greatly increased absorption by unfilledpolymers, enabling a highly controlled melting through the thickness ofplastic materials that do not have any absorbing agents and can betransparent in the visible wavelength range, i.e., “optically clear”parts such as polycarbonate or acrylic, without the need for any lasersensitive additives.

In the present invention, both workpieces absorb portions of the laserbeam (preferably a fiber laser), and the adjacent contact surfaces ofthese two work pieces are bonded together in subsequent cooling underpressure. The laser beam is directed at the contact surfaces atessentially a right angle through the mask so that the width of thelaser beam spot on the upper workpiece is limited by the slot width inthe mask. Thus, the melting of the upper part is limited by the width ofthe slot in the mask as the beam is moved. The workpieces are bondedtogether in subsequent cooling under pressure.

While particular embodiments and applications of the present inventionhave been illustrated and described, it is to be understood that theinvention is not limited to the precise construction and compositionsdisclosed herein and that various modifications, changes, and variationscan be apparent from the foregoing descriptions without departing fromthe spirit and scope of the invention as defined in the appended claims.

The invention claimed is:
 1. A laser welding system comprising: a laser beam source configured to emit a laser beam having a wavelength of about 2 microns; a first workpiece made of thermoplastic material that is optically transparent and partially permeable to said laser beam; a second workpiece made of thermoplastic material that optically transparent and partially permeable to said laser beam; a pair of clamping plates positioned and configured to engage opposite sides of said first and second workpieces when said first and second workpieces are adjoining each other with contacting surfaces, said pair of clamping plates being positioned relative to the laser beam source such that both said first and second workpieces partially absorb said laser beam to cause heating and melting in respective portions of said first and second workpieces; an actuator configured to urge at least one of said pair of clamping plates toward the other one of said clamping plates to press said first and second workpieces together; a transparent glass plate; a mask on the opposite side of said first workpiece from said second workpiece, said mask being impermeable to said laser beam and forming a slot for passing said laser beam to the portion of said first workpiece exposed by said slot, so that heating and melting of the material of said first and second workpieces is limited to the width of said slot, said mask being positioned on said transparent glass plate to absorb heat from said first workpiece to cool the portions of said first workpiece not exposed to said laser beam; and a drive unit configured to move said laser beam to illuminate said slot.
 2. The laser welding system of claim 1, wherein each of said first and second workpieces lacks any absorbing agents.
 3. The laser welding system of claim 2, wherein each of said first and second workpieces is composed of a polycarbonate or acrylic.
 4. The laser welding system of claim 1 wherein said laser beam has an optical fiber beam.
 5. The laser welding system of claim 1, wherein said laser beam is substantially perpendicular to said mask.
 6. The laser welding system of claim 1, wherein said mask is chrome plated on glass.
 7. The laser welding system of claim 1, wherein each of said first and second workpieces is an unfilled polymer that is can be optically transparent, and said laser beam is partially absorbed by said polymer.
 8. The laser welding system of claim 1, wherein said mask is a reflective-coated photo mask on a bottom surface of said glass plate facing said first workpiece.
 9. The laser welding system of claim 1, wherein said mask is on a top surface of said glass plate opposite a bottom surface thereof, said bottom surface facing said first workpiece. 